Since there has been a lack of explanations for it, I'll say what I've experienced.
While casting Magic attacks (haven't tried Ranged, so dunno if this is the case as well), enemies that are within casting range normally cause me to run towards them randomly, rather that stay in the spot I casted the attack at. They don't move at all.
This movement seems to be really buggy, as it makes me walk through certain objects. I got stuck in one of the rubble piles in Armadyl while trying to cast Blood Barrage on one of the enemies. The same effect happened at Kree, when we were against that wall. We just walked right through it trying to cast a Magic Attack.